It is recommended you run this project use gradle rather than your IDE; this is of particular importance if you are getting linkage errors due to multiple xml-api jar versions on the classpath when using IntelliJ.
To use gradle, the predefined run and runPages tasks can be used.
You can configure the test scripts to use the HtmlUnit browser emulator (the default) or a real browser via an appropriate driver. There are some comments in the source and build files to give you hints for several browsers but also see the Selenium WebDriver documentation for other browsers or more details.